The clinical diagnosis of proximal myopathy is usually straightforward. Proximal muscle wasting and weakness is easily demonstratable.
**The weakness is bilateral and usually symmetrical.
**The sensory is always INTACT!
The causes that you need to consider are:
1. Muscular dystrophy
**Duchenne/ Becker muscular dystrophy (look for pseudohypertrophy of calf muscle
** Fascioscapulohumeral dystrophy (look for facial muscle weakness)
**Limb girdle muscular dystrophy
2. Inflammatory muscle disease
** Polymyositis (muscle may be tender)
** Dermatomyositis (Facial heliotrope rash and Gottron's sign)
** Cushing's syndrome
**ALL will have obvious external features
4. Metabolic myopathies
** Hypo/hyperkalemia, Hypo/hypercalcemia
The above patient with proximal myopathy also has features of Cushing's syndrome. Remember that Cushing's syndrome can cause proximal myopathy but also do not forget that steroid use for polymyositis/dermatomyositis can also cause Cushing's syndrome.
How do you differentiate polymyositis, dermatomyositis and inclusion body myositis ?
Polymyositis Dermatomyositis Inclusion body myositis
Sex F>M F>M M>F
Age Adult Childhood & Adult Elderly > 50 years old
Rash Yes No No
Distribution of weakness Prox > Distal Prox > Distal Prox = Distal,
CK incr (up to 50x) N or incr (up to 50x) N or mildly incr (<10
Muscle biopsy amyloid deposits
immunosupressive Yes Yes No or minimal
EMG myopathic myopathic myopathic/neuropathic